Package ::= Header Imports Body

Header ::=
Header ::= package Name

Imports ::=
Imports ::= Imports Import

Import ::= import Name

Body ::=
Body ::= Body Element

Element ::= var Name
Element ::= var Name = Expr

Expr:$ ::= SumExpr

SumExpr:$ ::= MulExpr
SumExpr ::= SumExpr + MulExpr

MulExpr:$ ::= MoleculeExpr
MulExpr ::= MulExpr * MoleculeExpr

MoleculeExpr:$ ::= AtomicExpr
MoleculeExpr ::= ( Expr )

AtomicExpr:$ ::= Name
AtomicExpr:$ ::= Integer
